1. Field of the Invention
This invention relates to a mode decision circuit for detecting the mode used in producing an FM signal and, more particularly, to a method and apparatus for detecting whether the FM signal, which may be reproduced from a record medium, was recorded in a first mode occupying a first frequency band or a second mode occupying a second, different frequency band. Although this invention admits of a broad range of applications, it will be described in the environment of a video tape recorder wherein at least one component of the recorded video signal may be recorded in one or another frequency band.
2. Description of the Prior Art
In a typical video tape recorder (VTR), a composite video signal normally is recorded with a chrominance component whose original frequency is converted to a relatively low frequency region and with a luminance component that is frequency modulated. Video signal levels in the luminance component thus are represented by a range of frequency modulations. For example, a maximum, or peak white video level may be represented by a maximum frequency and the amplitude of the usual horizontal synchronizing signal, known as the sync tip level, is represented by a minimum frequency. The remaining video picture information is represented by frequencies within this range.
It is appreciated that improvements in horizontal resolution and picture quality can be achieved by broadening the range of minimum to maximum frequencies that the FM luminance signal may occupy. Accordingly, it has been proposed to use a carrier frequency higher than that used heretofore for FM luminance signal recording and to enlarge the frequency range, or deviation, used to represent video picture information in the luminance component. This can best be appreciated by recognizing that in a typical recording operation in the so-called 8 mm format, the carrier frequency of the FM luminance signal varies from a low of 4.2 MHz to represent the sync tip level to a high of 5.4 MHz to represent the white peak level. Improvement in picture quality is expected if this FM range is broadened and shifted such that the sync tip level is represented by, for example, 5.7 MHz and the white peak level is represented by, for example, 7.7 MHz. This latter recording range of the FM luminance signal is referred to herein as the "high band" mode of recording, whereas the aforementioned range of 4.2 MHz to 5.4 MHz is referred to as the "low band" mode of recording.
It is expected that some VTR's have the capability of recording a video signal (specifically, the FM luminance signal) in either the high band or the low band. Of course, whichever band is selected for recording also is selected for a playback operation. However, it may turn out that a video tape is recorded on another VTR; and a user may not be readily aware of the particular recording mode that was used. Nevertheless, for proper video signal reproduction, the VTR used to reproduce the previously recorded signals should be matched to the VTR which recorded those signals, and thus, it is desirable to provide some means for selecting a high band or a low band reproducing mode. For example, the operating characteristics of the usual FM demodulator, low pass filter, playback amplifier, or deemphasis circuit should be selected to accommodate either a high band or a low band FM luminance signal. While a manual mode change-over switch might satisfy this objective, it often is preferred to provide mode selection automatically and not rely upon accurate selection by a user of the VTR. Automatic band selection is particularly advantageous when a video tape has several programs recorded thereon, some of which having been recorded in the low band mode and others having been recorded in the high band mode.
One proposal for automatically detecting whether the reproduced FM luminance signal was recorded in the high band mode or in the low band mode is comprised of a band pass filter tuned to a particular frequency component normally included in a low band FM luminance signal. For example, the filter may be tuned to the particular frequency which represents the sync tip level (for example, a frequency approximately equal to 4.2 MHz) if the FM luminance signal is recorded in the low band mode. If this particular frequency is detected, a mode identifying signal indicative of the low band mode is produced. Conversely, if this particularly frequency is not detected, as when the FM luminance signal was recorded in the high band mode, the mode identifying signal is correspondingly indicative of that fact.
Another automatic mode detecting circuit which has been proposed heretofore includes two band pass filters: one tuned to the frequency included in the low band mode which represents the sync tip level and the other tuned to the frequency in the high band mode which represents the sync tip level. For example, the low band sync tip frequency may be approximately equal to 4.2 MHz and the high band sync tip frequency may be approximately equal to 5.7 MHz. A low or high band indication is produced as a function of which filter produces an output signal. To improve reliability, the outputs of these filters are compared to each other, resulting in a mode identifying signal of, for example, high or low amplitude depending upon which filter output signal is greater.
The aforementioned proposals suffer from the drawback of erroneous mode detection which may be caused by side band components passed by the low band filter. For example, if the FM luminance signal is recorded in the high band mode, the lower side band component of that signal representing video signal information may be of a frequency sufficiently close to the frequency to which the low band filter is tuned as to be passed by that filter. As a result, the low band filter produces an output signal when, in fact, the FM luminance signal occupies the high band. Accordingly, the mode detecting circuit produces an erroneous indication that the FM luminance signal had been recorded in the low band mode when, in fact, it occupies the high band. This erroneous indication may be produced even when the improved embodiment which compares the outputs of the low band and high band filters is used. In that case, the low band filter output derived from the lower side band of the FM luminance signal may exhibit a greater amplitude than the high band output which represents the sync tip level.
In an attempt to prevent the aforementioned erroneous mode identification, it has been further suggested that the FM luminance signal be supplied to the mode detecting circuit only during the interval that the synchronizing signal is present. According to this further suggestion, a gate circuit is enabled by a gate pulse which coincides in time with the synchronizing signal interval. Hence, the gate circuit is opened only when the synchronizing signal is present and, thus, the lower side band component of other useful video information (i.e. non-synchronizing information) is inhibited from reaching the low band filter when the FM luminance signal occupies the high band.
However, the proposed use of a gate circuit suffers from the disadvantage of relying upon the demodulated FM luminance signal to provide the synchronizing signal from which the gate pulse is derived. If the FM demodulator or other FM processing circuits are not properly adjusted to match the particular mode in which the FM luminance signal had been recorded, the FM signal will not be demodulated properly and, thus, a correct gate pulse will not be produced. For example, if the FM processing circuits are adjusted to match the high band mode, but the reproduced FM luminance signal actually occupies the low band mode, the low band FM signal will not be properly demodulated. This means that if the recording mode has not yet been determined, or if its initial selection is incorrect, proper FM demodulation may not be achieved and, thus, the input gate circuit might not operate properly.